This invention relates to a fuel supply control method for an internal combustion engine at deceleration, and more particularly to a method of this kind which is adapted to vary the range of an operating region of the engine wherein the fuel supply to the engine is interrupted, in response to the atmospheric pressure.
When supply of fuel to the engine is carried out while the intake pipe absolute pressure is low at deceleration of the engine with the throttle valve fully closed, a large quantity of unburnt fuel is emitted together with the exhaust gases, badly affecting the fuel consumption, emission characteristics, etc. of the engine. Also, in an internal combustion engine having a device for purifying the exhaust gases, like a three-way catalyst, a large quantity of unburnt fuel emitted together with the exhaust gases can cause burning of the bed of such exhaust gas purifying device, thereby increasing the emission of detrimental exhaust gases. A method for preventing the inconveniences such as described above is widely known, which carries out interruption of the fuel supply to the engine, i.e. fuel cut, while the engine is operating in a predetermined operating region at deceleration.
According to this known method, if the determination as to whether or not the engine is operating in the above predetermined operating region is made on the basis of the throttle valve opening when the engine speed is high, it can happen that fuel cut is not carried out even when the intake pipe absolute pressure is low enough for fuel cut to be carried out, resulting in the above-mentioned inconveniences. Therefore, it has previously been proposed by the assignee of the present application in Japanese Provisional Patent Publication (Kokai) No. 57(1982)-191426 that when the engine speed is high, the above predetermined operating region be determined on the basis of the intake pipe absolute pressure as well as the engine speed. More specifically, the predetermined operating region is set such that fuel cut is effected depending upon the engine speed even when the throttle valve is not fully closed, so long as the intake pipe absolute pressure is lower than an absolute pressure line assumed with no load on the engine but higher than another absolute pressure line corresponding to the maximum allowable bed temperature of the three-way catalyst below which the temperature of the three-way catalyst rises to an abnormal extent.
However, even if the above predetermined operating region of the engine is determined solely on the basis of the engine speed as well as the intake pipe absolute pressure, as proposed by the above method, without taking account of variations in atmospheric pressure encompassing the engine, the phenomenon can occur that the above-mentioned absolute pressure line assumed at no engine load is moved toward a lower absolute pressure side as the ambient atmospheric pressure decreases, when the engine is operated in a low atmospheric pressure condition, such as at a high altitude, thus reducing the range of the predetermined operating region, i.e. the fuel cut effecting region, of the engine. That is, while the engine is operated in a low atmospheric pressure condition with no engine load applied thereon, the detected intake pipe absolute pressure can assume a value lower than the absolute pressure value for determining whether or not fuel cut should be carried out. On such an occasion, if the accelerator pedal of the engine is stepped on to increase the engine speed while the engine is at idle, fuel cut can be carried out in the course of an increase of the engine speed, causing hunting of the engine rotation and other inconveniences. Although these inconveniences may be overcome by setting the fuel cut determining value of absolute pressure at a lower value for stable operation of the engine in a low atmospheric pressure condition, the range of the fuel cut effecting region is undesirably reduced, thereby hindering improvements in the fuel consumption, emission characteristics, etc. of the engine.